1. Field
This invention relates generally to the treatment of vegetable storage facilities and, in particular, to potato storage facilities with aerosols of CIPC.
2. State of the Art
Potatoes are frequently stored from harvest time for a number of months until spring or, often times, the following summer. Typically, the potatoes are stored in storage facilities which are ventilated and humidified. Air circulation is maintained inasmuch as potatoes undergo respiration when stored which gives off CO.sub.2, other chemicals and heat. It has long been known that unless specific steps or techniques are employed after storage that potatoes will sprout within a few months of initial storage and render the whole pile of potatoes interlocked and useless. Storage at temperatures of from about 42.degree. to 45.degree. F. is generally practical to minimize sprouting.
It is known in the art to apply sprout inhibitors of various types to potatoes to prevent sprouting during storage. One of the earlier patents relating to this is the Plant patent, U.S. Pat. No. 3,128,170, related to a method for applying isopropyl-N-chlorophenylcarbamate (CIPC) to a potato storage facility. As noted in the Plant patent, CIPC is a solid at room temperature which is generally dissolved in polar solvents such as propylene glycol and, more recently, methanol. A typical weight of CIPC in solution is about 78% of the weight of the solution for commercial products using methanol as the solvent. A solution of CIPC was desired for forming aerosols of CIPC as well as for handling.
More recent patents relating to the application of CIPC to a storage facility are Sheldon and Morgan, respectively, U.S. Pat. Nos. 4,226,179 and 4,887,525. Sheldon involves a process for ultrasonically vaporizing a solution of CIPC, while the Morgan patent relates to an improved technique for moderating the air flow within a storage facility to get better distribution of CIPC aerosol produced from solvent-based system.
Sheldon refers to the possibility of applying non-solvent based CIPC. No example of such technique is given in Sheldon nor is there any suggestion as to how such a technique would be accomplished. Sheldon indicates that a solvent may be necessary in order to keep the chemical liquid in the spray nozzle. (Col. 4, lines 50 et seq.) The temperature range indicated by Sheldon for the CIPC is 70.degree. F. to 250.degree. F. (Col. 5, lines 1 through 13). The melting point of pure CIPC, however, is about 104.degree. F. At Col. 9, lines 5 et seq., Sheldon indicates that the CIPC present should be at least 60% by weight and preferably 75% by weight of the chemical feed material with the remainder being solvent. Sheldon does not indicate that compressed air at about 70 psig. fed to the spray nozzle of his device be heated (Col. 10, lines 20 et seq.). The device of Sheldon utilizes a large quantity of air; 5000 cfm. is introduced into his misting device (Col. 10, lines 25 et seq.).
Sheldon notes that thermal fogging tends to produce large droplets of CIPC, cause degradation of the CIPC and warm the stored potatoes, which may promote bacteria growth.
The technique utilized in the Morgan patent involved thermal fogging prior to introduction of the fog of CIPC in the circulating air stream of a storage facility. Thermal foggers which have been commercially used are constructed similar to that illustrated in FIG. 1, wherein a propane flame burns within a hollow pipe (combustion chamber) which is enclosed by another generally cylindrical enclosure. It is into this outer enclosure that the solvent based CIPC is introduced. The solvent based CIPC is frequently introduced near the distal end of the combustion chamber with the solvent solution of CIPC being blended with the combustion gases emanating from the combustion chamber. This results in the solvent generally being evaporated and the CIPC being converted into a mixture of vapor (gas) and particles of CIPC, both liquid and solid particles.
The products of combustion exiting the combustion chamber are generally oxygen poor, so much of the methanol solvent is not burned. Thus, it evaporates and often decomposes to formaldehyde and formic acid, both of which are toxic. The products of combustion form a reducing atmosphere in a storage facility and further create an overpressure from the large volume of gases entering the facility. The reducing atmosphere causes the potatoes to be stressed, resulting in some of the starch being converted to sugars. Potatoes having a high sugar content yield french fries which are dark brown in color when cooked, especially at the tips of the french fries. This is generally undesirable and reduces the value of such stored potatoes. An over pressure results in much of the treatment chemical being vented from the storage facility.
As the Morgan patent noted, one of the problems had been that the CIPC tended to collect on the fans of the air circulation system of the storage facility as well as on the vent pipes and other portions of the facility. CIPC is not really effective for treating potatoes unless it is in contact with the potatoes, that is, deposited directly on the potatoes. The prior art thermal fogging system introduces into the potato storage facility all the products of combustion of the propane gas burner as well as evaporated methanol, or such other solvent, including decomposition products of methanol such as formaldehyde, formic acid and the like. Given that storage facilities are maintained at relatively low temperatures, in the neighborhood of about 40 to 50.degree. F., these products, methanol, formic acid, formaldehyde, and the like, can liquify (condense) within the facility and can also be deposited on the potatoes. Since this can happen, it can also create a vapor pressure of these products within the storage facility long after a sprout inhibition treatment has occurred. Thus, a facility can be rendered unsafe for personnel to work in for quite some time. Although thermal fogging with thermal foggers of the type illustrated in FIG. 1 has been done for a long time, and storage sheds so treated have been relatively free of sprout growth, nevertheless the method is inefficient in its application of CIPC, i.e., CIPC decomposes to some extent, contaminates the storage facility with toxic, undesirable material, and is lost by venting.
Both Sheldon and Morgan involve methods and apparatus which introduce large volumes of gases, air or combustion products into a storage facility. This creates an overpressure within the facility and causes venting and loss of CIPC from the facility.
The technique of forming aerosols, i.e., a stable fog, of herbicides, pesticides, etc. has conventionally involved the use of solvents or carriers. In U.S. Pat. No. 2,460,792 of Pabst et al., the technique of adding a mixture of oils to obtain a stable aerosol is disclosed. A principal reason for the use of solvents with CIPC and similar sprout inhibitors apparently has been to accommodate the application of the sprout inhibitor as an aerosol and to facilitate handling of liquids by applicators.
While the Sheldon patent suggests the forming of a liquid particle "fog" by ultrasonic means, the technique has apparently not been practiced commercially and the patent is devoid of any instruction as how this is done from a non-solvent system. Current field techniques for commercial application of CIPC has been by fogging of a solvent solution of CIPC via a thermal fogger of the prior art type, frequently using moderated fan speed as taught by the Morgan patent.